The invention relates to a record carrier suitable for thermomagnetically writing and magneto-optically reading information. The record carrier comprises a non-magnetizable substrate which bears an amorphous layer of a rare-earth metal/iron alloy having an easy axis of magnetization perpendicular to the plane of the layer. The invention also relates to an optical memory device including such a record carrier. (A rare-earth metal is defined as an element having an atomic number from 57 to 71 inclusive).
Record carriers as described above are known from Netherlands Patent Application No. 7508707, which has been laid open to public inspection. This Application discloses a record carrier having a layer of iron-gadolinium, with approximately 40 atomic percent gadolinium, and 60 atomic percent iron, which has been deposited on a substrate by thermal evaporation in a vacuum. Thermomagnetic writing takes place by locally heating the layer, for example by means of a focused laser beam, to the Curie temperature of the alloy while the layer is in a magnetic field, and then cooling the layer. The direction of magnetization of the heated area of the layer reverses under the influence of magnetic stray fields of adjacent nonheated areas. (When the direction of magnetization reverses, an external magnetic field is also used sometimes which is directed opposite to the field in which the layer is present).
A disadvantage of the known record carrier is that the structure of the a morphous material changes irreversibly at comparatively low temperatures (from 100.degree.-150.degree. C.). The properties of the layer, notably the magnetic properties, also change. In the long run, this process leads to crystallization of the material. Since in practice the material repeatedly experiences a rise in temperature each time information is written, so as to bring it near the Curie temperature of the material, the above-described crystallization process is most undesirable.